The present invention relates to a new and improved construction of tunnel furnace or kiln for fabricating plate-shaped flat structures, in particular printed circuit boards, coated at both sides or faces with a material which can be cured.
Generally speaking, the tunnel furnace of the present development, useful for the fabrication of plate-shaped flat articles or structures, especially printed circuit boards, which have been coated at both faces with a material which can be cured or hardened, is of the type comprising a conveyor device for the flat articles or structures and a heating device enclosing a hot air infeed device.
During the fabrication of printed circuit boards, it is known to apply to both sides of a base plate or substrate a curable material, for instance, an epoxy resin and to cure such within a furnace or kiln. This is accomplished by initially curing the material layer at the one side or face of the plate and thereafter the layer of material at the opposite side of such plate. The initially cured material layer therefore must pass twice through the heating furnace, and thus, during its second pass is again exposed to thermal action, causing a further curing of the applied material layer. Hence, this material layer tends to cure or harden much too intensely. This in turn renders more difficult, if not impossible, further processing such as photographic development of the coated plate or board or the like.
Now if, in order to avoid such phenomenon, the furnace temperature and the velocity of travel of the plates through the furnace are chosen such that the first material layer, passing twice through the furnace, during its first pass is only partially cured and then during its second pass cured to such an extent that a subsequent further processing of this material layer of the plate which has passed twice through the furnace is still possible, then the second material layer which is applied still remains tacky and possesses an irregular surface which is caused by solvent inclusions. If it is desired to also avoid this drawback, then the heating efficiency of the furnace and/or the throughpass velocity, during the second throughpass of the plates must be changed in relation to their first throughpass. This renders extremely difficult any continuous operation and also requires a correspondingly great expenditure in equipment and controls.